1. Technical Field
The present disclosure relates to a display. More particularly, the present disclosure relates to a blue phase liquid crystal display.
2. Description of Related Art
In recent years, for improving the display quality of a liquid crystal display (LCD), blue phase liquid crystals with rapid response are gradually valued, in which the blue phase represents a liquid crystal phase between the isotropic phase and the cholesteric phase and only exists in a narrow temperature range of about 1° C.
The blue phase mainly has three different types, which are the first blue phase (BP I), the second blue phase (BP II) and the third blue phase (BP III), in which liquid crystals of the first blue phase and the second blue phase are in a cubic form, and liquid crystals of the third blue phase are in an amorphous form and exist in a temperature higher than those of the other two types of blue phase.
FIG. 1a and FIG. 1b are schematic diagrams showing a lattice structure and disclination lines of the first blue phase liquid crystal respectively. FIG. 1c and FIG. 1d are schematic diagrams showing a lattice structure and disclination lines of the second blue phase liquid crystal respectively. As shown in FIG. 1a and FIG. 1c, a basic unit of the lattice structure of each of the first and second blue phase liquid crystals is a double twist cylinder (DTC) 100; that is, the double twist cylinders therein are arranged perpendicular with each other. Moreover, the first blue phase liquid crystal has a body-centered cubic (BCC) structure, and the second blue phase liquid crystal has a simple cubic (SC) structure. The disclination lines 102 of the first blue phase liquid crystal and the second blue phase liquid crystal are shown in FIG. 1b and FIG. 1d. Different from nematic liquid crystals, smectic liquid crystals and isotropic liquid crystals, the first blue phase liquid crystals and the second blue phase liquid crystals are shown as platelet texture patterns when being viewed under a polarizing microscope.
On the other hand, the positive blue phase liquid crystal generally uses a lateral electric field induced by electrodes to change its refractive index, thereby enabling the blue phase liquid crystal to generate the change of the bright/dark state after light passing therethrough. The positive blue phase liquid crystals are isotropic in an ideal state and the refractive index change (i.e. Δn) thereof is 0 (zero) in the condition without a lateral electric field from the electrodes. In addition, the positive blue phase liquid crystals in the ideal state are normally black, which herein means that light cannot pass the blue phase liquid crystals when no voltage is applied thereto. Conversely, when the lateral electric field is applied to the positive blue phase liquid crystals, the positive blue phase liquid crystals are anisotropic and the refractive index thereof is changed (i.e. Δn>0), such that the light can pass through the blue phase liquid crystals and the bright state can be shown.
Recently, the blue phase liquid crystal layer mainly consists of components including blue phase liquid crystal molecules and chiral dopants. The chiral dopant can be used for inducing blue phase liquid crystal molecules to form aforesaid double twist cylinders.
Based on the material characteristics of the blue phase liquid crystals, the lattice period of the blue phase liquid crystals follows a function of the wavelength, and accordingly, a selective Bragg reflection would occur base on an incident light with different wavelengthes. In other words, the blue phase liquid crystal molecules have a specific reflective band due to the material characteristics. The reflective band of undoped blue phase liquid crystal molecules fall in the visible light spectral range. However, the undoped blue phase liquid crystal molecules encounter a light leakage problem from the reflective band in a dark state.
In order to reduce the light leakage problem and increase the contrast ratio, chiral dopants of high concentration are usually added to the blue phase liquid crystal layer currently used in a conventional blue phase liquid crystal display device. FIG. 2 is a schematic diagram illustrating a relationship between reflective luminance and wavelength of the reflective light form the positive blue phase liquid crystal layer when the chiral dopants of high concentration are added to the blue phase liquid crystal layer. As shown in FIG. 2, the reflective band 102 of the conventional blue phase liquid crystal layer is usually in an ultraviolet light range 106 located outside the visible light range 104 after the chiral dopants is added at a high concentration.
The conventional blue phase liquid crystal layer can shift the reflective band 102 of the blue phase liquid crystal layer into the ultraviolet light range 106 by adding the chiral dopants at high concentration to reduce the light leakage problem. However, adding chiral dopants at higher concentration will result in the increase of the operating voltage required by the blue phase liquid crystal layer. This induces a difficulty in operating the blue phase liquid crystal layer.